An extrusion die is used to extrude molten thermoplastic material into a relatively thin film or sheet. Typical extrusion dies have a flow channel formed between a pair of die bodies and a pair of die lips. The die lips are positioned on the downstream end of the die bodies to form an outlet or exit orifice of the flow channel. In operation, molten polymer flows through the flow channel from an inlet opening provided on the upstream end between the pair of die bodies to the outlet orifice provided on the downstream end between the die lips. For example, a conventional coat hanger die has an inlet, an inlet manifold, a generally triangular or “coat hanger”-shaped preland channel, a final land channel, and a die exit formed between a pair of die lips.
In practice, sheet manufacturers are often required to produce sheets of different thicknesses to meet various customer demands. For this reason, an extrusion die can have adjustable lip gap spacing. The distance between the lips of the extrusion die can be adjusted by the manufacturer to control the size of the die exit. In turn, this controls the thickness of the sheet exiting the die.
To allow the spacing between the die lips to be adjusted, the die lips may be formed of separate structures from the die bodies that are then attached to the die bodies. For example, in some configurations, the die lips are bolted to the die bodies with multiple bolts positioned at spaced intervals along the length of the die bodies. In use, an operator may be required to loosen all of the bolts holding the die lip to the die body, move the die lip relative to the die body, and then retighten all the bolts. To ensure that molten polymer does not leak through the gap between the die lip and the die body, the operator is usually required to torque each bolt holding the die lip to the die body down with a specific amount of force. This process of loosening the bolts, adjusting the die lip, and carefully retightening the bolts can be time consuming, necessitating several hours of downtime to adjust the die lips.